As part of commissioning a hydro turbine generator after major assembly and alignment, a verification test must be performed to show that the generator rotor is centered in the stator. Generator rotors can vary, for example, from less than 6 meters in diameter to over 15 meters in diameter with an air gap of about 1 centimeter up to about 2 centimeters. The air gap is typically maintained within 10% of nominal, which requires that the rotor be centered to within, for example, from about 1 millimeter to about 2 millimeters of the center of the stator for the typical range of air gap size. On generators with high pressure lift thrust bearings, an air gap sensor is temporarily attached to the rotor, such as at a rotor pole, and the rotor is spun by hand. Output of the sensor is recorded during rotation of the rotor and an indication of the completion of a revolution is provided, either on the same channel as the sensor output or on a second channel, to define the limits of each rotation.
The stator is perceived as a non-continuous surface with stator teeth and winding slots because the sensor is responsive to electrically conductive material, resulting in variation in the magnitude of the output signal of the sensor. Depending on the type of sensor, the points at which the rotor is closest to the electrically conductive material of the stator (the tips of the stator teeth) are either minima or maxima of the output signal. The true shape of the stator surface can be shown by plotting the data points that correspond to the ends of the stator teeth, which are typically the minima or lowest values of the of the raw output signal.
However, because of the manual rotation of the extremely heavy rotor, the speed of rotation varies, and the frequency of the minima of the output signal of the sensor also varies. As a result of the varying speed, a once per turn event provides no way to detect how far the rotor has rotated for each recorded signal value and therefore the location of each winding or tooth can not be detected using a simple timing or sample count method. To overcome this, the recorded output is reviewed and the minimum value of the output for each tooth of the stator is determined manually. The manual determination is cumbersome and time consuming and leaves room for error. Accordingly, there is a need in the art for an easy, convenient, automated way to determine how close to centered a rotor is relative to a stator that allows for variable rotational speed of the rotor relative to the stator.